Method for testing or recording servo signal on perpendicular magnetic recording media

ABSTRACT

An object is to automatically detect the positions of spike noise, create a distribution diagram, and perform pass/fail decisions. The cross-correlation function of the signal waveform from a magnetic head  1  and a reference waveform simulating spike noise is used in extraction of spike noise. The number of peaks in the cross-correlation function exceeding a threshold value is counted, and quantitative evaluation of spike noise is performed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for testing perpendicular magneticrecording media, and a method for recording servo signals onperpendicular magnetic recording media.

2. Description of the Related Art

In perpendicular magnetic recording, there exist a mode which usessingle-layer perpendicular magnetic recording media, and a mode whichuses double-layer perpendicular magnetic recording media. In the lattercase, there is the problem of spike noise peculiar to double-layerperpendicular magnetic recording media. Perpendicular magnetic recordingmedia have, below the recording layer comprising a perpendicularmagnetization film having a high coercive force to retain information, asoft magnetic layer serving as a return path for the recording magneticfield from the recording head during recording. In general, when thereexists a soft magnetic film of size equal to that of the magneticrecording media, a plurality of magnetic domains are formed so as toreduce the magnetostatic energy. This is readily understood bypractitioners of the art. When such magnetic domains exist, a strongmagnetic field emanates from the neighborhood of the magnetic domainwalls of these magnetic domains, so that each time a playback headpasses over such areas, a spike-shape output is observed. This is whatis generally called spike noise. When information or control informationused by the magnetic disk device is written at a position where spikenoise exists, the waveform of the playback signal is disturbed. Whensuch disturbances occur, the playback of information, or correctoperation of the magnetic disk device, is impeded. Spike noise isdescribed for example in the Journal of Applied Physics, Vol. 57, No. 1,pp. 3925-3927 (1985). A description of amplitude modulation of playbacksignals due to spike noise is given for example in the conference digestof The Fifth Perpendicular Magnetic Recording Conference (PMRC), PMRC2000, on pages 35 and 36.

Spike noise can be suppressed by, for example, using the techniquedescribed in the Journal of the Magnetics Society of Japan, Vol. 21, No.S1, pp. 104-108 (1997). However, even in the case of media in whichspike noise is suppressed, prior to use in a product it is necessarythat the presence of spike noise be checked, and that, should spikenoise exist, the state of distribution thereof be understood. Also, itis of course necessary to understand the distribution of spike noisewhen developing double-layer perpendicular magnetic recording media.

One of the simplest methods for observing the state of spike noise indouble-layer perpendicular magnetic recording media is a method using aspin-stand and oscilloscope. A spin-stand is a device used in magneticrecording experiments, comprising a spindle motor for rotating magneticrecording media, a mechanism to cause a magnetic head to seek aprescribed radial position on the media, an amplifier to operate thehead and amplify the output signal thereof, and an amplifier for use inrecording. Broadly defined, a spin-stand may further comprise ameasurement instruments necessary to perform the magnetic recordingexperiments, such as to evaluate the basic properties of recordingmedia, as well as a computer to control them. In such cases, theequipment is often called a read/write tester.

When an oscilloscope is used to observe head output, if there existmagnetic domains in the soft magnetic layer of the media, spike noisewill of course be observed. The spindle motor normally outputs an indexsignal (rotation origin signal), and if this is used as a trigger forthe oscilloscope, it is possible to determine, for a given radius, theangular position from the rotation origin at which the spike noiseappears. Either an analog or a digital storage type oscilloscope may beused. Although this method is suitable for obtaining informationrelating to the spike noise at a given radial position, it is notsuitable for grasping the state of spike noise over the entire mediasurface. This problem can be resolved by using a digital storageoscilloscope and a computer. As one example, a procedure is described inthe IEEE Transactions on Magnetics, Vol. 29, No. 6, pp. 3742-3744(1993). That is, while varying the radial position at which observationsare made, a digital storage oscilloscope is used at each radial positionto detect the head output, which is sent to a computer; the amplitudeinformation is converted into a brightness modulation signal, effectingtwo-dimensional visualization. By using this method, the state of spikenoise can easily be grasped intuitively.

The above-described prior art enables either observation of spike noisepositions at a given radial position, or two-dimensional visualizationof spike noise. These results are qualitative, and so can be used inqualitative evaluations in the research stage; but they cannot be usedfor pass/fail decisions in production processes. In research anddevelopment also, automatic data processing was not possible. Among theabove conventional technologies, the latter example has the problemthat, due to the large quantity of data obtained, if the results are tobe stored without further processing, a large amount of the recordingcapacity of a storage device would be consumed. Also, if spike noisetraverses servo signals, a prominent external disturbance is added tothe servo signal read by the head, giving rise to the problem of greatlyreduced tracking precision.

On the other hand, while the above-described prior art enabledevaluation of the distribution, amplitude, waveform, and similar ofspike noise, when a signal was actually recorded at the position ofspike noise, it was not possible to evaluate the effect on the playbacksignal. Ultimately, the problem for practical purposes might be thatsignals cannot be recorded correctly, or that recorded signals cannot beplayed back correctly. In actuality, it has been found that there are atleast two types of influence of spike noise on playback signals:baseline shifts, and amplitude modulation. The effects of each of theseon the performance of a magnetic recording device differ.

SUMMARY OF THE INVENTION

The present invention was devised in light of these problems, and has asan object the provision of a method for the quantitative evaluation ofspike noise. This invention has as a further object the automaticdetection over a broad range of the media of the effects on playbacksignals of spike noise, discrimination of the types of such effects, andquantitative evaluation of the magnitude of these effects. Thisinvention also has as an object the provision of a method for recordingservo signals such that traversal by spike noise is minimized.

In the soft magnetic layer of the perpendicular magnetic recordingmedia, there may exist magnetization states which are not observed asspike noise, but which may disturb the shape of the envelope of theplayback signal. This is distinguished from spike noise, but has asimilar effect on playback signals; below, for convenience, suchmagnetization states are also referred to simply as spike noise.

In order to attain the above objects, the perpendicular magneticrecording media testing method adopted in this invention ischaracterized in having a step in which the output signal waveform froma magnetic head loaded at a prescribed radial position of perpendicularmagnetic recording media, rotating at a prescribed velocity, is capturedand stored; a step in which the cross-correlation function between thestored output signal waveform and a reference waveform simulating spikenoise is calculated; and a step in which the number of peaks in thecross-correlation function exceeding a preset threshold value iscounted.

The perpendicular magnetic recording media testing method of thisinvention is characterized in comprising a step in which the operation,in which the output signal waveform from a magnetic head loaded at aprescribed radial position of perpendicular magnetic recording mediarotating at a prescribed velocity is captured and stored, is repeated aplurality of times while changing the loaded radial position; a step inwhich, at each radial position, the cross-correlation function betweenthe stored output signal waveform and the reference waveform simulatingspike noise is calculated; a step in which the coordinates on the mediaof peak positions at which the cross-correlation function exceeded thepreset threshold value are stored; and, a step in which a decision ismade as to whether the coordinates of peak positions exist continuouslyfor a preset length or longer on the media.

As the reference waveform simulating spike noise, a waveform having asingle positive or negative peak, or a waveform having one or morepositive and negative peaks each, can be used. Also, the perpendicularmagnetic recording media can be rotated at a fixed angular velocity, anda reference waveform which peak width adjusted according to the radialposition of the magnetic head can be used as the reference waveform.

Another perpendicular magnetic recording media testing method of thisinvention is characterized in comprising a step in which a magnetic headis loaded at a prescribed radial position of perpendicular magneticrecording media comprising a soft magnetic underlayer, and signals arerecorded at a prescribed frequency; a step in which recorded signals areplayed back; and a step in which the type of effect on the playbacksignal of spike noise appearing in the playback signal is discriminatedby means of the envelope shape of the playback signal.

By means of the playback signal envelope shape, the amplitude modulationcomponent contained in the playback signal waveform which is equivalentto spike noise, or the baseline shift in the playback signal, can bediscriminated. Amplitude modulation can be discriminated using ahigh-pass filter and envelope detector, or using a homodyne detector.

This perpendicular magnetic recording media testing method can comprisea step in which the amplitude modulation component is determined fromthe envelope shape of the playback signal, a step in which the amplitudemodulation component is eliminated from the playback signal, and a stepin which the baseline shift is determined from the playback signal withthe amplitude modulation component removed.

In calculating the actual cross-correlation function, because both thereference waveform and the observed waveform are given by a discretesystem, the correlation function is also a correlation function of adiscrete system. Because the length is finite for both referencewaveforms and observed waveforms, if the respective lengths are 2 M andN, then the reference waveform and observed waveform are given by thefollowing respective sequences. Here M and N are both integers.

Reference waveform: {u_(i)} (i: integer, −M≦i≦M−1)

Observed waveform: {X_(j)} (j: integer, 0≦j≦N−1)

In what follows, for convenience, both u_(i) and x_(j) are assumed to bezero-valued when there is a reference outside the above defined ranges.Also, it is assumed that N>>2M.

Here, a cross-correlation function sequence {y_(j)} normalized using thereference waveform is used. This sequence is defined by the followingequation. $\begin{matrix}{y_{j} = {\frac{1}{U}{\sum\limits_{k = {- M}}^{M - 1}\quad{x_{j + k}u_{k}\quad\left( {{j:{integer}},{0 \leq j \leq {N - 1}}} \right)}}}} & (1)\end{matrix}$

Here U is the maximum value of the autocorrelation function sequence forthe reference waveform, and is given by the following equation.$\begin{matrix}{U = {\max\left( \left\{ {\sum\limits_{k = {- M}}^{M - 1}\quad{u_{k}u_{j + k}}} \right\} \right)}} & (2)\end{matrix}$

Here the function max({z_(i)}) is a function which determines themaximum value of the sequence {z_(i)}. Through this normalization, inorder that U≦1 in nearly all cases, the threshold value should also beconsidered in the range from 0 or above to less than 1, so that itbecomes easier to determine the threshold value.

By using the correlation function with the reference waveform, itbecomes possible to reliably detect even spike noise with smalleramplitude than the spike noise detected by direct peak detection fromraw observed waveforms. The advantages of using the correlation functionare explained using FIGS. 2A and 2B. FIG. 2A shows raw data, and FIG. 2Bshows the cross-correlation function. In FIG. 2A, the line appears thickdue to the influence of medium noise and other noise. Clearly, in FIG.2B the effect of noise is greatly suppressed. In FIG. 2A, when thethreshold value is set to 0.1 or lower in order to detectsmall-amplitude spike noise, and peak detection is performed, numerousnoise peaks are erroneously discriminated as spike noise. Conversely, itwould appear that the threshold value must be set to at least 0.1 orhigher in order to prevent erroneous discrimination; but in this case,small-amplitude spike noise is overlooked. In FIG. 2B, even if thethreshold value is set as small as approximately 0.05, nearly allsmall-amplitude spike noise can be detected, without erroneousdiscrimination of noise. Here, as the reference waveform, a dipulsewaveform like that of the following equation (3) was used.$\begin{matrix}{y_{i} = {{\mathbb{e}}^{- {(\frac{i + 50}{20})}^{2}} - {{\mathbb{e}}^{- {(\frac{i - 50}{20})}^{2}}\quad\left( {{j = {- 100}},{- 99},{{- 98}\quad\ldots}\quad,97,98,99} \right.}}} & (3)\end{matrix}$

Also, media noise is eliminated, so that erroneous detection due tosmall ripples is prevented. FIGS. 3A and 3B show an enlarged display ofspike noise and its cross-correlation function. FIG. 3A shows the rawdata, and FIG. 3B shows the cross-correlation function. The raw data hasa place, slightly to the right of the maximum point, at which the peakseems to be split by noise. Here also, in simple peak detection there isa large possibility of erroneous discrimination of the existence ofanother peak. On the other hand, in FIG. 3B noise is suppressed, and sothere is no such concern.

There is considerable freedom in determining the threshold value. Themethod for determining the most suitable threshold value will differwith the objective of evaluation. If the objective is to detect evensmall-amplitude spike noise, of course the threshold value should be setas low as possible. However, if set too low, the probability that noisewill be erroneously discriminated as spike noise is increased. If, at agiven media diameter, the number of peak detections is determined whilereducing the threshold value, it is found that the number of detectionsincreases sharply when erroneous discriminations due to noise begin tooccur, as shown in FIG. 4; the threshold value should be set at as smalla value which is still larger than the value at which this increaseoccurs. This method can be used to automatically determine the mostsuitable threshold value. For example, when the number of peakdetections simply exceeds a certain value, or when the rate of change ofthe number of detections exceeds a certain value, it is concluded thaterroneous discrimination is occurring.

Because differences may occur in the results obtained using differentreference waveforms, the reference waveform must be determined withconsideration paid to the soft magnetic layer material, film thicknessand layer structure of the media being tested, and to the electricalcharacteristics of the head and playback system. The simplest referencewaveform, with the broadest range of application, is a single-pulsewaveform having a single peak; one example is indicated by the followingequation (4). $\begin{matrix}{y_{i} = {{\mathbb{e}}^{- {(\frac{i}{20})}^{2}}\quad\left( {{i = {- 50}},{- 49},{- 48},{{- 47}\quad\ldots}\quad,47,48,49} \right.}} & (4)\end{matrix}$

In actuality, the curve shape is not so important, and similar resultsare obtained if a Gaussian waveform, a Lorentzian waveform, or similaris used. But in the case of the single-pulse waveform, the width of thereference waveform is an important parameter. Of course the closer thewidth of the reference waveform is to the width of a peak in theobserved waveform, the larger is the correlation function value. Hencewhen using a single-pulse waveform, by selecting a waveform such thatthe width of the reference waveform is as close as possible to the peakwaveforms of actual spike noise, appropriate detection can be performed.

Often the spike noise actually observed is like that shown in FIG. 5,although there are differences depending on the soft magnetic layermaterial and the layer structure of the media. Hence by using adipulse-type waveform as the reference waveform, having onepositive-direction and one negative-direction peak each as shown in FIG.6, the noise suppression effect is larger, and more effective detectionis possible. The waveform indicated in the above equation (3) is onespecial example falling within this category. This waveform ischaracterized by the following four parameters: half-maximum width a ofthe peak 1, half-maximum width b of the peak 2, distance d between peaks1 and 2, and amplitude ratio e/f of peaks 1 and 2. However, amplitudesare normalized with e or f, whichever is larger, set equal to 1.

It is also possible to optimize the shape parameters of the referencewaveform with respect to the spike noise waveforms in the media to betested. For a single-pulse waveform, this is performed by determiningthe half-maximum width of the reference waveform such that the peakvalue of the cross-correlation function between the single-pulsewaveform and an actually captured spike noise waveform is maximized. Fora dipulse waveform, half-maximum widths are determined for both thepeaks 1 and 2 by the same method as that for single-pulse waveforms. Inthis case, e/f is determined from the peak values of the respectivecorrelation functions. Finally, the value of d which maximizes the peakvalue of the correlation function between the dipulse waveform and thespike noise waveform is determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a summary drawing showing one example of perpendicularmagnetic recording media testing equipment of this invention;

FIGS. 2A and 2B are drawings which explain the advantages of using acorrelation function;

FIGS. 3A and 3B are figures showing in enlargement an example of spikenoise and the cross-correlation function of same;

FIG. 4 is a figure showing the relation between the threshold value andthe number of peak detections;

FIG. 5 is a figure showing a typical example of spike noise;

FIG. 6 is a figure explaining a dipulse-type waveform;

FIG. 7 is a figure explaining the process of spike noise detection;

FIG. 8 is a figure showing an example of the result of calculation ofthe spike noise distribution;

FIG. 9 is a figure explaining the film structure of perpendicularmagnetic recording media used in experiments;

FIG. 10 is a figure showing an example of perpendicular magneticrecording media testing equipment capable of pass/fail decisions;

FIG. 11 is a figure explaining the procedure for pass/fail decision;

FIG. 12 is a figure explaining perpendicular magnetic recording mediatesting equipment which can be connected to a network;

FIGS. 13A and 13B are figures explaining the directional dependence ofthe spike noise DC erasure magnetic field;

FIG. 14 is a figure explaining the manner in which playback signals aremodulated by spike noise;

FIG. 15 is a figure showing an example of another configuration ofperpendicular magnetic recording media testing equipment;

FIG. 16 is a figure explaining the procedure for acquisition of envelopewaveform data for a playback signal;

FIG. 17 is a figure showing an example of the configuration of adiscrimination circuit capable of eliminating baseline shift;

FIGS. 18A, 18B and 18C are figures showing an example of observation ofbaseline shift due to spike noise;

FIGS. 19A, 19B and 19C are figures showing an example of observation ofamplitude modulation due to spike noise;

FIG. 20 is a figure showing an example of the configuration of adiscrimination circuit used to discriminate baseline shift and amplitudemodulation;

FIG. 21 is a figure explaining the procedure for judging media quality;

FIG. 22 is a figure showing an example of the configuration of adiscrimination circuit using homodyne detection;

FIG. 23 is a figure showing an example of the configuration of adiscriminator for simultaneous acquisition of upper and lower envelopes;

FIG. 24 is a summary figure showing an example of a servo signalrecording device of this invention;

FIG. 25 is a summary figure showing an example of the arrangement ofservo signals;

FIG. 26 is a figure showing an example of traversal of a servo signalseries by a spike noise series;

FIG. 27 is a figure showing an example of avoidance of traversal of aservo signal series by a spike noise series, by changing the phase forstarting servo signal recording;

FIG. 28 is a figure showing an example of traversal, by two adjacentspike noise series, of servo signal series at two consecutive locationson the same track; and,

FIG. 29 shows an example of avoidance of the traversal by spike noiseseries of servo signal series at two consecutive locations on the sametrack, by changing the phase for starting servo signal recording.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, embodiments of this invention are explained, referring to thedrawings. In the following figures, parts with the same function areassigned the same symbols, and redundant explanations are omitted.

First Embodiment

FIG. 1 is a summary drawing showing one example of perpendicularmagnetic recording media testing equipment of this invention. Thetesting equipment of this example is controlled overall by a computer 7.The computer 7, a digital storage oscilloscope 3, and a spin-standcontrol device 6 each incorporate a bidirectional bus interface, bywhich means these devices are interconnected. Of course anotherconnection method, such as for example connections using a serial bus,may also be employed.

Testing operations are performed according to a test programincorporated in the computer 7. The test program first causes thespindle motor 4 to rotate at a prescribed revolution rate, via thespin-stand control device 6. Then, the head stage 5 is driven to loadthe head 1 at a prescribed radial position on the double-layerperpendicular magnetic recording media 8. The output signal from thehead 1 is amplified by the head amplifier 2, and is input to the inputterminal of the digital storage oscilloscope 3. The digital storageoscilloscope 3 acquires the amplified head output over a desired periodof time, using the index signal from the spindle motor 4 as a triggersignal, and stores the output in memory. The stored head output waveformis transferred to the computer 7 by a read instruction issued by thetest program. This series of data acquisition operations is performedeach time the head 1 is moved to each radial position within a presetradial position range. Normally, the head 1 is often moved in the radialdirection at equal intervals; but of course movement at unequalintervals is of course also possible. Hence at the time of completion ofdata acquisition operations, information is stored in the memory of thecomputer 7 concerning the two-dimensional distribution of spike noise.

Next, the test program detects spike noise from the stored waveformdata, and creates a two-dimensional distribution figure based on theresults. Detection of spike noise is performed for each waveformobtained from each radial position. Hence spike noise positions areshown for a radius and for a time from the index signal, that is, anangle from the rotation origin on the disk. A head output waveformcaptured by the digital storage oscilloscope is of course discrete alongthe time-axis, and so in the following explanation, unless stipulatedotherwise, correlation functions and similar are all discrete along thetime axis. Also, when mounting the test disk on the spindle, if the markformed on the edge of the disk during disk fabrication by the substrateretaining fixture of the sputtering system is aligned with the spindlerotation origin, the position of spike noise can be represented as anabsolute position on the disk.

Spike noise detection is performed according to the procedure shown inFIG. 7. The degree of similarity between a reference spike noisewaveform, assumed in advance, and the head output is calculated, andbased on this spikes are detected. Here, a cross-correlation function isused to calculate the degree of similarity between the reference spikenoise waveform and the obtained head output waveform (correlationfunction calculation routine). Peak detection is then performed for thecalculated cross-correlation function, and those features recognized aspeaks are judged to be spike noise waveforms (peak detection routine).The quantity of data of the correlation function thus obtained isapproximately equal to the amount of original data, and typically isseveral megabytes. This large quantity of data is inconvenient, both forstorage and for use. However, although depending on the purpose forwhich the data is used, in general there are many cases in which, afterspike noise detection, a high angular axis resolution is unnecessary.Here the angle-direction axis is divided into, for example, 500 to 1000sections per revolution as necessary, the number of spike noise peaksexisting in each of the sections is counted, and results are displayedusing colored points corresponding to the number (angular axisresolution adjustment routine). Through such operations, the amount ofdata can be greatly reduced.

One feature of this method is the fact that, in the process ofcalculating the cross-correlation function, the noise amplitude arisingfrom media noise and from electrical circuits, and unrelated to spikenoise, is suppressed, so that the effect of these noise sources isgreatly reduced during peak detection.

FIG. 8 shows an example of a spike noise distribution diagram actuallyobtained using the equipment and procedure described above.

The double-layer perpendicular magnetic recording media evaluated hasthe film structure shown in FIG. 9, and was fabricated by the followingmethod. DC magnetron sputtering was used to form a Co-3 at % Ta-5 at %Ze film to a thickness of 400 nm, as a soft magnetic layer 102, on aglass substrate 101 of diameter 65 mm. On top of this, a Co-22 at %Cr-14 at % Pt film was formed to thickness 20 nm as the recording layer104, and on this a C film was formed to thickness 5 nm as a protectivefilm 105.

The magnetic head used in evaluations employed a magnetoresistanceeffect element as the playback element; the shield gap was 0.2 μm, andthe track width was 1.3 μm. This head and the above-mentioneddouble-layer perpendicular magnetic recording media were mounted on thespin-stand, and experiments were conducted. In these experiments, therotation velocity of the media was held constant at 3000 rpm.Measurements were conducted at 100 μm intervals between an innercircumference of radius 20 mm, and an outer circumference of radius 28mm. The head position was adjusted such that the skew angle was 0° atall radial positions.

The width of the spike waveforms of spike noise varied depending on thematerial and thickness of the soft magnetic layer, the head used, theangle made by domain walls and the head, and other factors; but whenusing an amorphous Co—Ta—Zr soft magnetic material film of thicknessapproximately 400 nm as the soft magnetic layer, the width was typically100 to 300 μm. Hence when using media of diameter 65 mm, in order tocapture such spike noise waveforms, the requirements of the samplingtheorem require that the sampling interval be smaller than 50 μm. Inthis case, the maximum circumference length is approximately 200 mm, andso in order to capture the data of one media circumference at thisradius, a minimum of 4000 points are required. When considering thenumber of points necessary to adequately represent the features of thereference waveform, 100,000 sampling points are necessary percircumference. If the data volume per sample is one byte, then if datais acquired in 0.1 mm intervals from radii of 20 mm to 28 mm, the totaldata volume is approximately 7.7 megabytes, a huge amount. As explainedabove, depending on the purpose for which the data will be used, afterdetection of spike noise, there are many cases in which high resolutionof the angle axis is unnecessary. For example, when judging pass/failfor the media based on whether there exists a radius, in a fixed rangeor beyond, in which there are no encounters with spike noise, theresolution in the angle direction clearly is not important after thespike noise position has been calculated.

In this embodiment, one object is to obtain a diagram of the broaddistribution on the media of spike noise; hence after detecting spikenoise, the angular-axis direction is divided into 500 sections, thenumber of spike noise occurrences in each section is counted, and whenone or more exists, they are represented by a black point. FIG. 8 is adiagram obtained in this way. In the case of FIG. 8, whereas theoriginal data volume was approximately 7.7 megabytes, the volume of datain FIG. 8 is approximately 40 kilobytes, since the angular-directionresolution is 500 points. As the reference spike noise waveform used inthe process of obtaining FIG. 8, a single-pulse waveform, expressed bythe above equation (4), was used.

Peak detection was performed by a method which regards as peaks thosemaxima in the spike correlation function for which the spike correlationfunction before and after the maximum exceeds a set threshold value forat least a fixed length of time.

As the most general formats for data storage following spike noisedetection, storage as distribution coordinate data, in a state in whichthe resolution used in drawing FIG. 8 is reduced, or as image data likethat of FIG. 8, is possible. In addition, the peak position coordinatesof spike noise can be recorded as binary data or as text data. Here, ifthe data is represented using the same angular axis-direction resolutionas when calculating the position coordinates of spike noise from thecross-correlation function, particularly when there are infrequentappearances of spike noise, positions can be recorded using even smallerdata quantities than in the methods described above, and with highposition resolution maintained. Also, in addition to the position of thespike noise, the amplitude of the peak waveform (spike correlationfunction) judged to be spike noise, as well as the half-maximum width ofthe peak waveform, the amplitude of the head output waveform, and otherdata can also be recorded as necessary.

By assuming a specific shape such as a dipulse-type waveform like thatexpressed by equation (3) instead of the single-pulse shape of equation(4) as the reference spike waveform, only spike noise exhibiting awaveform of a specific shape is detected, so that types of magneticdomain walls can for example be classified.

As explained above, the absolute values of such reference waveformparameters as the shape and width are not so important, and it issufficient to use either a waveform observed in advance, or a predictedwaveform. For example, if conditions are the same as in the actualobservation example above, by using a single-pulse type referencewaveform such as that described by equation (4) with a half-maximumwidth of 100 μm, nearly all spike noise can be detected. However, in theabove example, the cross-correlation function is calculated using thesame reference spike noise waveform, even at different radii. Hence inthis method, because the angular velocity of media rotation duringmeasurements is fixed, the effective length of the reference spike noisewavelength changes considerably from the inner circumferences to theouter circumferences. If the measurement radius extends over a broadrange, for example from 15 mm to 30 mm, it may not be possible to ignorethis effect. In order to avoid such circumstances, the reference spikenoise waveform is calculated for each radius such that the length of thereference spike waveform is constant at each radius. That is, the widthand shape of the reference waveform is stipulated in advance, usingactual dimensions. As one simple example, there is a method in which,based on the optimized reference spike noise waveform at a given radius,the values of each of the elements of the reference spike noise waveformarray at another radius are calculated by interpolation. Using thismethod, a reference spike noise waveform suited to each radius can becalculated easily and rapidly, even when the reference spike noisewaveform is not given by a numerical formula.

Hence the actual procedure is as follows. First, the width and shape ofthe reference waveform are stipulated using actual dimensions. Next, theprocedure described above is used to capture the head output waveform ateach of the radii within the test range. Then, the correlation functionis calculated for each radius, and peak positions are determined. Atthis time, the reference waveform determined by the method explainedabove is used as the reference waveform for each radius.

As test results, in addition to a display of the spatial distribution ofspike noise like that of FIG. 8, the number of spike noise occurrencesencountered by the head for each measurement radius can be representednumerically, and the total number of spike noise occurrences observed,the frequency distribution for a peak amplitude of the correlationfunction, and other results can be calculated. Pass/fail decisionconditions can be set for each of these independently or in combination.

It is certain that much spike noise is due to leakage magnetic fieldsfrom the walls of magnetic domains formed in the soft magnetic layer.However, a not insignificant amount of the spike noise detected is onlyobserved as points in a two-dimensional distribution diagram. Theseoccurrences are thought to arise not from simple magnetic domains, butfrom faults in the soft magnetic layer. Regardless of the origin, theextent of the effect in actual magnetic disk devices of spike noiseobserved as points and spike noise observed as lines in atwo-dimensional distribution diagram is different, and so the two shouldbe classified differently. That is, whereas point-shape occurrencesaffect only a few tracks, line-shape occurrences affect an extremelylarge number of tracks. These results can also be used in finalpass/fail decisions of tested disks.

Numerous methods can be used to discriminate between the two. Below, oneexample is explained. First, an exhaustive list of all the coordinatesof pixels at which spike noise exists is created. One element is takenfrom this list, and the list is examined to determine whether thecoordinates of pixels neighboring this pixel are on the list. Theelement thus taken is removed from the list. If such a neighboring pixelis discovered, the discovered pixel is also deleted; and the list isagain examined to determine whether the coordinates of pixelsneighboring the discovered pixel are recorded on the list. This isrepeated until neighboring spike noise coordinates are no longerdiscovered; and whether the occurrence is point-shaped or not isdiscriminated by whether the number of repetitions exceeds a stipulatedvalue. The coordinates of spike noise judged to be connected in a lineare recorded on a new list, for each connected line. The above processis repeated until the original list is empty. When extracting a newelement from the original list, if it is anticipated that the spikenoise will exhibit a radial distribution, possibly because theanisotropy of the soft magnetic layer is directed in the disk radialdirection, this operation is begun from an innermost or from anoutermost circumference, and when searching for neighboring elements,priority is given to the radial direction. By this means, elementsneighboring in the circumferential direction are less likely to beconfused. At present, the criterion for finally judging an occurrence tobe a line-shaped spike noise series is 300 μm. This is because radialmovement steps during measurements are normally 100 μm, and because onecondition is that at least three points be neighboring, in order toreduce the probability of error.

In this embodiment, calculations to detect spike noise are performedafter acquiring all information across the entire disk; but obviouslycalculations can also be performed upon measurement at each radius.Also, in this embodiment, the calculations are performed by softwareemploying the standard calculation methods of a computer; of course,specialized hardware, digital signal processors, and similar can also beused. And, in this embodiment data is captured at each specified radius;of course, the disk surface can also be scanned helically, continuouslycapturing waveforms while continuously causing the head to seek newpositions on the disk.

In the above example, the angular velocity of media rotation was heldconstant; however, measurements can also be performed with the linearvelocity held constant. In this case, sampling intervals are alwaysequal even at different radii, so that the reference waveform width canbe held fixed. Also, because the head flying height does not change,there is the feature that the spike noise detection sensitivity does notchange with the radius, even over a broad range of measurement radii.

In FIG. 1 and subsequent explanations, for simplicity, evaluation ofonly one side of a disk is described; of course equipment used in actualproduction lines can be designed such that both sides of a disk areevaluated simultaneously. That is, two heads can be mounted on a headstage such that measurements can be performed simultaneously on bothsides of the disk, and two channels of the head amplifier and otherelectric circuitry are provided. Either a digital oscilloscope which, inaddition to the index signal input, has two or more channels is used, orelse a plurality of oscilloscopes, with equivalent performance, areused. In order to perform inspections more rapidly, of course aplurality of heads may be mounted per disk side, and each head may beoperated in parallel. In this case, the necessary number of circuits andmeasurement instruments are of course installed.

Second Embodiment

FIG. 10 is a figure showing in summary another example of inspectionequipment used in executing a perpendicular magnetic recording mediatesting method of this invention. The equipment configuration of thisembodiment is the same as in the first embodiment, but this embodimentdiffers in that a media quality decision function is added to the testprogram.

As one of the simplest criteria for decision of quality, a condition forpassing might be that the number of spike noise occurrences in aspecified radius, or the number of spike noise occurrences within aspecified radius range, be less than a fixed number. However, thedetails and conditions for quality decision will differ depending on thequality standards to be satisfied by the disk. It is possible to setdecision conditions by combining the results of analysis functionsdescribed in the first embodiment, so that an exceedingly great varietyof circumstances can be accommodated. Moreover, consideration is paidsuch that the details and conditions for quality decision can easily bymodified by changing the decision routines and decision conditions.

FIG. 11 shows a summary of the operation of the quality decisionroutines in the test program. Decision routines can be set correspondingto a plurality of decision details; the number and details can bemodified through simple revisions to the program. Decision routines 1through N judge quality referring to the spike noise distributionobtained as a result of measurements and the amplitude and other presetdecision details and conditions, and perform rankings according to theresults. In the synthetic decision routine, a synthetic decision methodis defined in terms of the priority of each decision detail andinterrelations of same; a final decision is made using the decisionresults of each decision routine, and a final ranking is assigned.

When using the inspection equipment of this invention in an actualproduction line, the equipment is of course combined with a robot devicewhich automatically changes disks. Classification of the spike noiseevaluation results between passing and failing is performed immediatelyafter this stage. In some cases, in addition to simple classificationinto passing and failing items, ranking may also be performed asnecessary based on the inspection results.

Third Embodiment

FIG. 12 shows still another example of testing equipment used inexecuting a perpendicular magnetic recording media testing method ofthis invention. In this example, the control computer 7 comprises anetwork interface, and instructions and data can be sent to and receivedfrom others computers, measurement equipment, output devices and similarover the network. The test program comprises functions for interpretingand executing external instructions sent from other computers or otherdevices. Here, external instructions are defined within the testprogram, and the test program can be instructed to execute or interrupttests, modify test conditions, and similar. Hence remote operations fromother computers over the network are possible. It is not necessary touse a dedicated network interface in configuring a network; thebidirectional bus interface with which the device shown in FIG. 1 isequipped can also be used. Through these features, operations such asthe following are possible as necessary.

A single control computer can manage a plurality of spike noise testingdevices in a manufacturing line or similar. Also, data management can beunified and made easier. Or, another computer can be made to handleoperations involving large loads such as spike extraction. By thismeans, the time required for evaluation can be shortened, andproductivity can be improved. Also, by replacing the computer of thespike noise testing equipment with a less powerful machine, costs can bereduced. And, the spike noise testing equipment need not be a dedicateddevice, but can also be used as a read/write tester.

When having another computer handle spike noise extraction or otheroperations involving large loads, the spike noise testing equipment andcomputer need not be connected by a network, and of course head outputdata can be transported between them by means of removable storage mediaor similar.

Fourth Embodiment

As mentioned in the first embodiment, depending on circumstances,phenomena apparently similar to spike noise such as thermal asperity maybe observed. Thermal asperity is a phenomenon in which pulse-shapepseudo-signals appear when minute protrusions on the disk and dustparticles make contact with the magnetoresistance effect head element.

The frequency with which thermal asperity is observed is not normally sovery high; but the measures which should be taken in response to thermalasperity arising from the disk surface shape and spike noise are ofcourse different, and so depending on the purpose of the testing, it maybe necessary to distinguish the two. This can be achieved by utilizingthe following characteristic of spike noise.

FIGS. 13A and 13B show an example of observation with an analogoscilloscope of part of the spike noise in media described above. Thetime-axis scale is the same 100 μs per grid for both FIG. 13A and FIG.13B. Both FIGS. 13A and 13B show the results of observation of spikenoise (three occurrences) in the DC-erased state, but the sign of thecurrent passed in the recording head during DC erasure was different. Asseen in FIGS. 13A and 13B, the waveform of the spike noise changes withthe sign of the current during DC erasure of the media, that is, withthe direction of magnetization in the recording layer. Although there issome change in the amplitude and waveform in many cases, the polarity ofthe spike waveform also is reversed according to the direction ofmagnetization. But in the case of thermal asperity arising from the disksurface shape and from dust, the waveform polarity is not inverted evenif the sign of the DC erasure current is reversed. Also, in many casesthe spike noise waveform is a dipulse-type shape having positive andnegative peaks, as seen in FIGS. 13A and 13B, rather than a single-pulsewaveform.

Hence the simplest method for discriminating spike noise and thermalasperity is as follows. At an arbitrary measurement radius position onthe disk, a measurement magnetic head is used to perform DC erasure byapplying a sufficiently large magnetic field in a given direction, andthen spike noise is evaluated by the method described above. At thistime, a dipulse-type reference waveform is used. Next, DC erasure isperformed at the same radius with the magnetic field reversed during theDC erasure, and spike noise is then similarly evaluated. A dipulse-typereference waveform is used, so that when the cross-correlation functionof the signal waveform and reference waveform is calculated, spike noiseis detected only once in the two measurements.

FIG. 14 shows the results of observation with an analog oscilloscope ofthe same location on the same media observed in this embodiment usingequipment combining the functions of a read/write tester, afterrecording a signal with a magnetization transition interval of 254 nm.The amplitude of playback signals is seen to be modulated at theposition at which spike noise was observed. After a spike noise positionis identified, the signal-to-noise ratio at the position and in thevicinity was measured, and it was estimated that the signal-to-noiseratio is reduced by approximately 6 dB compared with areas in whichthere is no spike noise.

Fifth Embodiment

FIG. 15 is a summary diagram showing another example of testingequipment used in execution of the perpendicular magnetic recordingmedia testing method of this invention. The testing equipment of thisinvention comprises a discriminator 9 to discriminate the types ofinfluence of spike noise on playback signals. The procedure for testingcan be broadly divided into a data acquisition stage and adiscrimination stage. FIG. 16 shows the procedure of the dataacquisition stage. Testing operations are performed according to thetest program incorporated in the computer 7. Below, the procedure isexplained together with the equipment operation.

The test program first rotates the spindle motor 4 at a prescribedrevolution rate via the spin-stand control device 6, and drives the headstage 5 to load the head 1 at a desired radial position of thedouble-layer perpendicular magnetic recording media 8. Signals are thenrecorded using a write current specified in advance, at a recordingwavelength specified in advance.

The playback output signals of the head 1 are amplified by the headamplifier 2, and after passing through the discriminator 9, are input tothe signal input terminal of the digital storage oscilloscope 3. Thediscriminator 9 is a circuit which detects the envelope of the playbacksignal, that is, which continuously detects the amplitude. Thisoperation is discussed further below. The digital storage oscilloscope 3acquires and stores in memory the amplified head output over a desiredtime, using as a trigger signal the index signal from the spindle motor4. The stored discriminator output waveform is transferred to thecomputer 7 in response to a read instruction issued by the test program.This series of data acquisition operations is performed each time thehead 1 is moved to one of the radial positions within a preset range ofradii. Normally, the head 1 is often moved by equal intervals in theradial direction, but of course can also be moved by unequal intervals.Hence when the data acquisition operation ends, information regardingthe envelopes of playback signals for each radius is stored in thememory of the computer 7.

The test program extracts the effects on the playback signal of spikenoise from the data acquired in the above data acquisition stage,discriminates the types of effect, and if necessary creates atwo-dimensional distribution diagram from the results. Extraction of theeffects of spike noise from the acquired data is performed for waveformsobtained at each radial position. Hence the positions of points at whichthere is an effect of spike noise are represented as a radius and a timefrom the index signal, that is, an angle from the rotation origin on thedisk. When mounting the disk for testing on the spindle, if the markformed on the edge of the disk during disk fabrication by the substrateretaining fixture of the sputtering system is aligned with the spindlerotation origin, the position of a point affected by spike noise can berepresented as an absolute position on the disk.

In this embodiment, calculations to extract the effect of spike noiseare performed after acquiring all the information across the disk; butcalculations may also be performed at each measurement radius. Also, inthis embodiment software employing standard computer calculation methodsis used in calculations; but dedicated hardware, a digital signalprocessor, or similar can also be used. And, in this embodiment data isacquired for each specified radius; of course the disk surface can alsobe scanned helically, continuously capturing waveforms whilecontinuously causing the head to seek new positions on the disk.

A plurality of configurations for the discriminator 9 are conceivable.The circuit of FIG. 17 is an example of the simplest circuitconfiguration. After amplifying the head amplifier output to anappropriate level using the amplifier 11, the output passes through ahigh-pass filter 12. By appropriately selecting the cutoff frequency ofthe high-pass filter 12, the baseline shift of the playback signal dueto spike noise can be eliminated. Although depending on the type ofmedia and on measurement conditions, when the head-media relativevelocity is 9 m/s, when using a second-order Bessel-type high-passfilter, a cutoff frequency of approximately 500 kHz to 1 MHz isappropriate. Here a baseline shift is a phenomenon in which the averagevalue of the playback signal is locally shifted in the positive ornegative direction compared with other areas, due to superpositioning onthe playback signal of a frequency component differing from the playbacksignal (normally, a component at a far lower frequency than the playbacksignal).

Next, the output of the high-pass filter 12 is input to the envelopedetector 13. If the time constant of the envelope detector 13 isselected appropriately with respect to the playback signal frequency,the envelope of the playback signal, that is, the amplitude signal canbe obtained. Hence by detecting a valley in the output waveform of thediscriminator, the amplitude modulation due to spike noise can beidentified. Here, amplitude reductions due to other media defects orother factors, and not caused by spike noise, may be simultaneouslydetected. However, in general, there is a tendency for the range on themedia over which the playback signal amplitude is modulated by spikenoise to be from tens of microns to hundreds of microns, much broaderthan for the case in which noise is due to loss in the recording film orsimilar. Hence while discrimination with absolute certainty is notpossible, modulation of the playback signal amplitude by spike noise canbe identified with a considerable degree of certainty. It is alsopossible to measure the spike noise distribution, and performdiscrimination referring to the results. However, from the standpoint offault inspection, if areas in which the playback signal amplitude isdecreased are to be recognized as defective, it is sufficient todiscriminate as defects areas in which the playback signal amplitude isreduced below a certain criterion, regardless of the cause, and sodiscrimination of the cause is not necessarily important.

The construction and principle of operation of an envelope detector isfor example described in detail on pp. 125 to 126 of CommunicationSystems by B. P. Lathi, published by McGraw-Hill Book Co., Sep. 1, 1977.However, the envelope detector used in this testing equipment need nothave the configuration described in the above publication, and may adoptanother form, so long as the envelope of the playback signal can bedetected. Moreover, a rectifying detector or similar can also be used.Also, in FIG. 17, the high-pass filter 12 may be positioned before theamplifier 11 instead.

FIGS. 18A, 18B and 18C show an example of the baseline shift of theplayback signal due to spike noise. The evaluated double-layerperpendicular magnetic recording media is that explained in FIG. 9. Themagnetic head used in evaluations employs a magnetoresistance effectelement as the playback element; the shield gap is 0.2 μm, and the trackwidth is 1.3 μm. This head and the above-described double-layerperpendicular magnetic recording media were mounted on a spin-stand, andexperiments were conducted.

FIG. 18A is the head output waveform in the DC-erased state, in which anarea near a spike noise occurrence is observed. FIG. 18B is the resultof observation at the sample observation place as in FIG. 18A, afterrecording a signal. The linear recording density was 100 kFCI. At theposition of spike noise, the playback signal baseline can be seen to beshifted gently along the spike noise waveform. FIG. 18C shows theresults for the same place as in FIG. 18A and FIG. 18B, observed afterpassing through a high-pass filter with a cutoff frequency of 500 kHz.In FIG. 18C, there is only some disturbance in the shape of the enveloperemaining at the spike noise position, and the effect of spike noise isalmost entirely absent. From this it is concluded that the effect ofspike noise on the playback signal is almost entirely a baseline shift.

FIGS. 19A, 19B and 19C show an example of observation of a spike whichaccompanies amplitude modulation of the playback signal. As in FIGS.18A, 18B and 18C, FIG. 19A is the result of observation of the DC-erasedstate, FIG. 19B of a playback signal after recording, and FIG. 19C ofthe playback signal after passing through a high-pass filter. Clearlythe amplitude of the playback signal after elimination of the baselineshift by the high-pass filter is reduced at the position of spike noise.

At the place where the playback signal is reduced, in general thesignal-to-noise ratio is lowered. Hence if the amount of reductionexceeds a certain amount, this may naturally be regarded as a defect. Asa criterion for this judgment, for example, a decrease by 10% or morecompared with the average output amplitude in an interval on the sametrack in which no amplitude modulation is seen may be regarded asaffecting the media performance, and a decrease exceeding 30% may beregarded as a defect.

On the other hand, if the shift amount of the baseline shift is lessthan a certain limit, the recording area can be used without problem.This limit amount depends on the disk drive design, and in particular onthe signal processing method used in the actual magnetic disk device.For criteria values currently in use, baseline shift amounts within 200%of the maximum output value (the output at low linear recordingdensities) can be accommodated. As one example of a method to handlebaseline shifts which are within such criteria, a high-pass filter maybe dynamically inserted at the stage at which a baseline shift exceedinga certain value is detected. Although insertion of the high-pass filterhas some effect on the channel performance, areas in which spike noiseexists can be used, so that the average recording density of the diskcan be made higher than if such areas could not be used. Also, in thecase of a channel in which the DC component is utilized, by handlingspike noise in a manner similar to thermal asperity, areas in whichspike noise exist can be employed as recording areas.

In this way, it can be seen that the effect of spike noise on playbacksignals should be handled differently according to the type of theeffect. It is also seen that handling should be different according tothe magnitude of the effect. Hence when making a pass/fail decision fora tested disk, by quantitatively evaluating the effect on the playbacksignal of spike noise, more accurate decisions become possible comparedwith decisions made simply on the basis of the number of spike noiseoccurrences or similar. For example, instances in which the overallactual effect of spike noise on playback signals is small, but a usabledisk is rejected due to a large total number of spike noise occurrences,can be avoided. Conversely, cases in which the number of spike noiseoccurrences is small, but these have a large effect on the playbacksignal, so a disk which is in fact unusable is accepted, can also beavoided. In other words, the manufacturing yield of media and magneticdisk devices can be raised.

Thus the test program makes pass/fail decisions based not only on thenumber of points at which baseline shift and amplitude modulation of theplayback signal due to spike noise occur, but also on the extent of theeffects of each. As one index for use in making a synthetic pass/faildecision for a tested disk, the average recording density is used. Thatis, the average recording density is calculated from the number ofdefects and areas which should be treated as defects, and from thenumber of areas which can be used, but with reduced performance, and theextent of the reduced performance. In addition to simple pass/faildecisions, it is also possible to classify tested media into a pluralityof grades, for example using the average recording density as an index.

Sixth Embodiment

A modification of the testing equipment shown in FIG. 15 is explained.The testing equipment of this embodiment has a discriminator withconfiguration different from that of the testing equipment explained inthe fifth embodiment; otherwise the configuration is the same.

FIG. 20 is a figure showing an example of the configuration of adiscriminator used in the testing equipment of this embodiment. In thisexample, there are two discriminator output channels. One of thechannels is the same as in FIG. 17; the other channel is that of FIG.17, but with the high-pass filter removed. The output of the lattersimply passes through the envelope detector 13, and so the outputcontains a baseline shift and amplitude modulation due to spikes. Hencein this output, the baseline shift and amplitude change appear as peaksand valleys. However, the reduction in amplitude and the negativebaseline shift both appear as valleys, and so the two types of effectcannot be discriminated. Hence, as explained in the fifth embodiment,amplitude modulation can be discriminated from the output on the sidepassing through the high-pass band filter 12, so that by comparing thetwo, baseline discrimination and amplitude modulation can bediscriminated.

If a digital storage oscilloscope having two or more input channels isused, the output of the two channels of FIG. 20 can be capturedsimultaneously, so that even if this discriminator is used, no largedifference in detection times occurs compared with the case in which thediscriminator of FIG. 17 is used.

FIG. 21 shows the procedure in the decision stage for separatelyevaluating the baseline shift and amplitude modulation due to spikenoise, and for using both these results in a synthetic evaluation ofmedia performance. In this case, the acquired data is of two types: anenvelope detector output series obtained by sampling the output of theenvelope detector, and an amplitude modulation data series obtained bysampling the envelope detector output after passing through a high-passfilter. The latter mainly comprises information on the playback signalamplitude, but in practice also comprises components other than theplayback signal amplitude, since the envelope detector is affected bymedia noise and other factors. Similarly, the former also comprisescomponents other than the playback signal amplitude and baseline shift.However, the extent of the effect of media noise is the same in both, soin this case, when calculating the baseline shift, the amplitudemodulation data series is simply subtracted from the envelope detectoroutput series. In the following, this is called the baseline shiftseries.

Points at which amplitude modulation occur are found by calculatingminima points, that is, valleys, in the playback signal amplitude foreach radius from the amplitude modulation data series. Detection ofvalleys is performed by searching for points at which the amplitude isdecreased by a fixed fraction or more from the average amplitude over afixed interval, and at which the derivative passes through zero. Bycomparing the extent and number of such discovered amplitude modulationpoints, for example, the media grade with respect to amplitudefluctuation points can be determined.

Points at which baseline shift occurs can similarly be determined fromthe baseline shift series. However, the baseline shift has a polarity,and so it is necessary to determine maxima and minima. The media gradewith respect to baseline shift can be determined similarly to theprocedure used for amplitude modulation. Finally, a synthetic pass/faildecision can for example be made based on both the amplitude modulationand the baseline shift evaluation results.

Seventh Embodiment

Below, another modification of the testing equipment shown in FIG. 15 isexplained. The testing equipment of this embodiment has a discriminatorconfiguration different from that of the testing equipment explained inthe fifth and sixth embodiments, but otherwise the configuration is thesame.

FIG. 22 is a figure showing an example of the configuration of ahomodyne detector which is a discriminator used by the testing equipmentof this embodiment. The details of the configuration and operation ofhomodyne detectors are explained in many references and elsewhere, andso are not discussed here (for example, see pp. 135 to 138 ofCommunication Systems by B. P. Lathi, published by McGraw-Hill Book Co.,Sep. 1, 1977).

Operation is briefly explained as follows. Here as previously, a seriesof “1”s is recorded on the media, so that the playback signal is acyclic signal. The carrier extraction circuit 71 generates a signalhaving a frequency synchronous with the playback signal. If a circuitemploying a phase-locked loop is used, a signal which is almostcompletely synchronous with the fundamental frequency (carrierfrequency) and phase of the playback signal can be obtained. Byinputting this signal and the playback signal into the mixer 72, andremoving unnecessary high-frequency components using a low-pass filter73, the playback signal envelope can be obtained under nearly idealconditions. Also, the playback signal baseline shift is also nearlycompletely removed from the output from this circuit. On the other hand,in a discriminator comprising an integrator circuit as in an envelopedetector, the capacitor is charged by the noise voltage, so that theobserved playback signal amplitude is larger than the actual amplitude.Hence this circuit is suitable for cases in which the degree ofamplitude modulation of the playback signal due to spike noise is to beevaluated quantitatively.

Eighth Embodiment

Still another modification of the testing equipment shown in FIG. 15 isexplained. The testing equipment of this embodiment has a discriminatorconfiguration different from that of the testing equipment explained inthe fifth, sixth and seventh embodiments, but otherwise theconfiguration is the same.

Many envelope detectors use an integrator and diode, so that envelopescan be detected separately on the positive and negative sides of theplayback signal. By comparing the envelopes on the positive and thenegative sides, the symmetry of the positive and negative sides of theenvelope can be evaluated, and in addition, the results can be used fordiscrimination of the baseline shift and amplitude modulation of theplayback signal. FIG. 23 is an example of the configuration of adiscriminator utilizing this feature. The output of the amplifier 11 isinput to the two envelope detectors 13, 13. The two envelope detectorshave the same configuration, and are adjusted to the same time constant.However, one detects the envelope on the positive side of the playbacksignal, and the other detects the envelope on the negative side. If theenvelope detector uses a diode and an integrator, it is sufficient toinvert the polarity of the diode of one of the detectors. Hence bytaking the sum of the outputs of the two, the baseline shift can beobtained; by taking the difference, the positions of amplitudemodulation can be obtained as peaks or valleys in the output signal.

Ninth Embodiment

In the prior art, when servo signals are recorded onto double-layerperpendicular magnetic recording media in which spike noise isdistributed radially, it was not possible to avoid the occurrence oftraversal of servo signals and spike noise. However, if the spike noisedistribution can be determined in advance, it is possible to recordservo signals while avoiding spike noise.

As a device for recording servo signals, FIG. 24 is a conceptual diagramof a servo track writer. For simplicity, part of the connections in thefigure are omitted, but the control device 208 executes supervisorycontrol over the entire system. That is, the servo signal recordingcontrol unit is also controlled by the control device. Prior torecording servo signals, the control device first determines the spikenoise distribution using the method and procedure described in the firstembodiment. The spike noise distribution thus determined is stored inmemory. As opposed to a spin-stand, in a magnetic disk device prior torecording servo signals, there is no means for detecting the headposition; therefore a rotary encoder 205 is added to the rotary actuator206 to detect the head position as a rotation angle. Of the informationfor the head position on the media, angle coordinates are detected usingthe index signal of the spindle, or using a rotation synchronizationpulse obtained based on a cyclic signal recorded and played back by aclock head 201. The rotary actuator 206 is rotated by the head positioncontrol unit, and the position of the magnetic head 203 is movedsequentially to record the servo signal. The recording and playbackamplifier 207 obeys instructions from the control device and the servosignal recording control unit to amplify magnetic head playback elementdriving and playback signals in order to drive the magnetic head. Whenrecording servo signals, the timing is always adapted to the rotationsynchronization pulse obtained based on the cyclic signal recorded andplayed back by the clock head 201, and servo signals are recorded on thedouble-layer perpendicular magnetic recording media 202 by the magnetichead.

Servo signals are recorded, for example, in positions such as thoseshown in FIG. 25. The arrangement of the servo signal regions in arcshapes is due to the fact that a rotary actuator is used for magnetichead seeking. The servo signal areas in FIG. 25 are represented bylines, but in actuality, of course, they have a certain length in thecircumferential direction. Hence when servo signals are written in suchan arrangement on media in which there is spike noise, traversal ofspike noise occurs with a certain probability. An example of suchtraversal is shown in FIG. 26. Writing of the servo signal begins fromthe position of the outermost circumference, shown in FIG. 26. At thistime, one cause for the traversal of spike noise areas by servo signalsis an unsuitable disk rotation angle (phase) when servo signal writingis begun. By modifying the phase with which writing of the servo signalis begun such that there is no traversal of the servo signal and thespike noise areas, as in FIG. 27, comparing the previously obtainedspike noise distribution with the servo signal position information,traversal of the servo signal series and the spike noise series isavoided.

Another method of preventing traversal of servo signal and spike noiseareas involves changing the servo signal arrangement such that there isno traversal, based on the spike noise distribution obtained in advance.However, the servo signal must span neighboring tracks, so that it isdifficult for the position to be modified significantly from one trackto another. Upon recording normally as shown in FIG. 28, by shifting thewriting phase of a servo signal series which traverses a spike noisearea, the spike noise is avoided. In this method, there occur regions inwhich the phase difference between servo signal series is larger thannormal. Hence the amount of change in the write phase of the servosignal series must of course be within the range in which reductions intracking precision in the region can be held within the allowable range.

Through the above method, servo signals can be recorded while avoidingspike noise. However, there are cases in which neither of the twospecific examples described above is alone sufficient. Hence as a morepractical method, the two above procedures may be combined. That is,first a write starting phase with the lowest probability of traversalfor an ordinary servo signal arrangement is selected, and then the writestarting phase for each servo signal series is adjusted.

Depending on the number of spike noise areas and their distribution, oron the number of servo signals per circumference, it may not be possibleto avoid traversal of servo signals by spike noise areas, as in theexample shown in FIG. 28. In the example of FIG. 28, there are a largenumber of servo signals per circumference, so that when there exists aradially-continuous spike noise series, traversal of one of the servosignal series is virtually certain. Also, in this example the intervalbetween two adjacent radially-shaped spike noise series and the intervalbetween servo signal series are essentially the same in a given radiusrange (from track A to track B). Both the spike noise series and theservo noise series have a certain width, so that the phenomenon oftraversal occurs with considerable probability. In such cases, dependingon the phase at which servo signal writing is begun, servo signals maybe traversed by spike noise areas at two consecutive places in thecircumferential direction, as in the example of FIG. 28. In such cases,a normal servo signal cannot be obtained at two consecutive places,giving rise to the problem of reduced tracking precision. By adjustingthe phase of the beginning of writing, or the phase of writing for aspecific servo signal series, such that spike noise areas do nottraverse servo signals at two consecutive places on the same track, theeffect of spike noise can be minimized. FIG. 29 is an example in whichtraversal of servo signals by spike noise areas at two consecutiveplaces on the same track on the media of FIG. 28 is avoided by shiftingthe phase at which writing of servo signals is begun.

In this embodiment, the index pulse of the spindle 204 is used as thedisk rotation origin. When using a motor which does not output an indexpulse, a clock signal can be recorded using the clock head 201, and theplayback pulses counted to create an origin signal; or, a laser probecan be used to determine the origin signal from the texture of thespindle-side surface.

According to another aspect of the present invention, the followingperpendicular magnetic recording media testing device and the servosignal recording method are realized.

(1) A perpendicular magnetic recording media testing device, comprising:

-   -   means for supporting and driving in rotation perpendicular        magnetic recording media;    -   means for playing back signals arising from the magnetization        state of said perpendicular magnetic recording media; and,    -   means for detecting amplitude modulation components from signals        played back by the means for signal playback.

(2) A perpendicular magnetic recording media testing device, comprising:

-   -   means for supporting and driving in rotation perpendicular        magnetic recording media;    -   means for playing back signals arising from the magnetization        state of said perpendicular magnetic recording media; and,    -   means for detecting the baseline shift of signals played back by        the means for signal playback.

(3) The perpendicular magnetic recording media testing device accordingto (1), wherein said means for detection of amplitude modulationcomponents comprises a high-pass filter and envelope detector, or ahomodyne detector.

(4) A servo signal recording method, for recording servo signals onperpendicular magnetic recording media, comprising:

-   -   a step in which the spike noise distribution in the        perpendicular magnetic recording media is determined, and is        stored in memory;    -   a step in which said stored spike noise distribution is compared        with servo signal arrangement information, and the starting        phase for writing of the servo signal which has the lowest        probability of traversal by spike noise areas for a normal servo        signal arrangement is selected; and,    -   a step in which the phase for writing each servo signal series        is adjusted such that spike noise areas do not traverse two        consecutive servo signals on the same track.

Whereas in the prior art the position of spike noise was observed onlyat a given radius position, or was rendered visible in two dimensions toobtain only qualitative results, by means of this invention quantitativeevaluations can be performed, and pass/fail judgments in manufacturingprocesses as well as automatic data processing in research anddevelopment are made possible. Also, it is possible to greatly reducethe memory and the storage device capacity necessary to store theresults of evaluations of spike noise. In addition to informationconcerning the position and number of occurrences and the amplitude ofspike noise, it is also possible to quantitatively evaluate the effectsof spike noise when signals are actually recorded. Through this, moreaccurate pass/fail decisions and classification are made possible, sothat as a result manufacturing yields of double-layer perpendicularmagnetic recording media and of magnetic disk devices can be improved.Moreover, servo signals can be recorded so as to avoid traversal byspike noise areas.

1. A perpendicular magnetic recording media testing method, comprising:a step in which the output signal waveform from a magnetic head loadedat a prescribed radial position of perpendicular magnetic recordingmedia rotating at a prescribed velocity is captured and stored; a stepin which the cross-correlation function between said stored outputsignal waveform and a reference waveform simulating spike noise iscalculated; and, a step in which the number of peaks of saidcross-correlation function which exceed a preset threshold value iscounted.
 2. The perpendicular magnetic recording media testing methodaccording to claim 1, wherein a waveform having a single peak, eitherpositive or negative, is used as said reference waveform simulatingspike noise.
 3. The perpendicular magnetic recording media testingmethod according to claim 1, wherein a waveform having at least onepositive peak, and at least one negative peak, is used as said referencewaveform simulating spike noise.
 4. The perpendicular magnetic recordingmedia testing method according to claim 1, wherein said perpendicularmagnetic recording media is rotated at a fixed angular velocity, and assaid reference waveform, a reference waveform with peak width adjustedaccording to the radial position of said magnetic head is used.
 5. Aperpendicular magnetic recording media testing method, comprising: astep in which the operations to capture and store the output signalwaveform from a magnetic head loaded at a prescribed radial position onperpendicular magnetic recording media, rotated at a prescribedvelocity, are repeated a plurality of times, changing the radialposition at which the head is loaded; a step in which thecross-correlation function of said stored output signal waveform and areference waveform which simulates spike noise is calculated for eachradial position; a step in which the coordinates on the media of thosepeak positions of said cross-correlation function which exceed a presetthreshold value are stored; and, a step in which a decision is made asto whether said peak position coordinates exist continuously on themedia for a length equal to or greater than a preset length.
 6. Theperpendicular magnetic recording media testing method according to claim5, wherein a waveform having a single peak, either positive or negative,is used as said reference waveform simulating spike noise.
 7. Theperpendicular magnetic recording media testing method according to claim5, wherein a waveform having at least one positive peak, and at leastone negative peak, is used as said reference waveform simulating spikenoise.
 8. The perpendicular magnetic recording media testing methodaccording to claim 5, wherein said perpendicular magnetic recordingmedia is rotated at a fixed angular velocity, and as said referencewaveform, a reference waveform with peak width adjusted according to theradial position of said magnetic head is used.